Inheriting texture from another layer
You can join all objects into one, go to edit mode, top ortographic view, select all, press U and choose "Project from view".

Load your image into an UV editor window, adjust the scale until it fits.

Then assigne a material with the image as texture.
You can select all bottom zones (walls) and assign another material to avoid stretching.

Then you can separate all by loose parts (P) if you need to.